Musnad Ahmad 26:571

Waki' narrated to us, Al-A'mash narrated to us, from Abdullah ibn Murrah, from Abu al-Ahwas, from Abdullah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Behold, I disassociate myself from every intimate friend (khalil) because of his intimate friendship. And if I were to take an intimate friend, I would have taken Abu Bakr. Indeed, your companion is the intimate friend of Allah, the Almighty and Majestic."